Magnetic material and process for its production



Patented July 2, 1935 UNITED STATES MAGNETIC MATERIAL AND PROCESS F ITS PRODUCTION Franz Duftschmid, Heidelberg, Germany, assignor to I. G. Farbenindustrie. Aktiengesellschaft, Frankfort-on-the-Main, Germany No Drawing- Application October 7, 1931, Serial No. 567,478. In Germany October 9, 1930 6 Claims.

hereinafter referred to as firon metals, are

known for example under the trade names of "Perminvar or Permalloy; these alloys may be employed with advantage in electrical engineering, especially weak current techniquev or communication art. These alloys have a comparatively high initial permeability and some of them have, in addition thereto, .the further important property that the permeability remains constant over a wide range of field strength. For example, a homogeneous alloy of the said kind may have the composition: 30 per cent of iron, 25 per cent of cobalt and 45 per cent of nickel.

I have now found that new metallic substances having the said magnetic properties in a particularly high degree are obtained by consolidating and interdifiusing mixtures of at least two finely divided iron metals under sufliciently moderate conditions that, while the initial metal particles certainly combine with each other to form an alloy, this alloy does not become homogeneous, but is still unhomogeneous in its micro-structure, the single constituents being present in different concentrations at different places. The said new alloys may also contain, in addition to the iron metals, further metals of the 4th to the 7th groups of the periodic system, as for example molybdenum, vanadium, chromium, manganese and titanium, and also silicon or aluminium may be present therein.

The new unhomogeneous alloys differ from the homogeneous alloys hitherto known in that the iron metals, i. e. nickel, iron and cobalt, which are capable of alloying completely with each other, are only incompletely diffused into each other, so that the products have a microstructure in which the concentration of the single components varies from place to place, as for example from 100 per cent downwards. These fundamental difierences between the two kinds of alloys become obvious when the surface of the alloys is treated with one of the usual strong etching agents such as aqua regia or ferric chloride solution. On microscopic examination homogeneous alloys then reveal a grain structure, but the different crystal grains have been attacked by the etching agent to about the same extent. In the case of the new unhomogeneous alloys, however, the action of the etching agents is entirely difierent, in that the single grains, de-

pending on their composition, are attacked with diflferent intensity. Thus, crystals rich in nickel (with or without cobalt) are attacked by the etching agents only with difiiculty, while crystals low in the said metals sufi'er strong attack. Accordingly, the diflerences in the depth of the etching are very great, in some cases even to such an extent that single crystal particles are completely dissolved out while others are scarcely attacked at all. The fundamental differences between the two types of alloys can often be rendered visible also by the tempering of a sample in air or oxygen, because the microstructure assumes different colors at different places in the new alloys, whereas the color is uniform in the known homogeneous alloys.

Alloys according to the'present invention are eminently suitable for employment in weak current technique and the communication art, especially in cases where a high initial permeability is required which should remain constant over a wide range of field strength, as for example for the preparation of cores for loading coils or for winding Krarup cables. Unhomogeneous alloys, the average composition of which is about from 20 to 90 parts of nickel and about to 10 parts of iron, respectively, and which are as free as possible from carbon, sulphur, phosphorus and oxygen are especially suitable.

I As has been pointed out above, the new alloys are prepared by carrying out the consolidation of the finely divided metals under sufficiently moderate conditions. The consolidation which is accompanied by incomplete interdifiusion of the different metals is eifected by the aid of elevated temperatures which should as a rule be at least 600 C., but should preferably not exceed about 1000 C., because at higher temperatures interdiffusion takes place very rapidly and there is accordingly the risk that homogeneous alloys of the kind hitherto usual are formed. The time during which sumcient interdifiusion takes place, varies with the circumstances of each particular case; it will therefore be necessary to control the progress of interdifiusion by taking test portions which may be examined by the etching or tempering methods described above. Generally speaking, at a given temperature the time of interdifi'usion varies with the size of the particles of the initial metals employed, with the nature of the initial metals, and also with the nature of the mechanical treatments such as rolling or forging, to which the mass may have been subjected before interdifiusion is effected. Thus, for example, when using metals which have been produced by the thermal decomposition of the corresponding metal carbonyl and the particles of which have a size of the order of from 10- to 10- millimetre, the pulverulent mass may be first rendered compact and consolidated to some extent by the action of mechanical pressure and moderately elevated temperatures, as for example 600 C. or more, at which no substantial interdifiusion occurs, whereupon the mass is moulded and/or shaped by mechanical treatment such as forging or rolling and then subjected to the interdiifusion treatment at higher temperatures; as has been pointed out above, temperatures above 1000 C. should be applied only for a short period of time, if at all. By the said mechanical treatment such as rolling or forging, the mass is 'sion, so that interdiifusion takes place the more readily, the more strongly the material has been kneaded.

Depending on the conditions employed during the consolidation treatment, either porous or non-porous masses are obtained as the final products. In the case of porous masses the inter-' diffusion takes place less readily, so that even temperatures of the order of from 1100 to 1300 C. may be applied for very long periods, for example 1 or 2 days without the interdiffusion proceeding too far. If the consolidation is carried out with the aid of mechanical pressure treatment, compact masses are obtained, and in such cases the'interdifiusion proceeds too far at 1100 or1300 C. already in the course of 10 or hours, respectively.

The heating for efiecting interdiifusion is preferably carried out in an inert or reducing atmosphere or in vacuo, in order to avoid oxidation of the metals. However, the treatment may be carried out in the ordinary atmosphere, especially in case the metal powders contain substances which are volatilized during the heating and thus act as protective gases preventing the access of the atmosphere; this is possible for example in the case of metal powders containing carbon and oxygen which are volatilized by the formation of oxidesof carbon.

The interdiifusion may also be influenced by employing fine. and coarse metal powders together. Metals which diffuse only with difficulty, are preferably employed in the form of very fine powders to facilitate interdifiusion, whereas too rapid interdifiusion of readily diflusible metals can be prevented by employing the said metals in a comparatively coarse state. Of course, the several components of a mixture may have different sizes of particles, whereby it is possible for example in the case of a three component alloy composed of the metals A, B and C, to effect much more rapid interdiffusion between the com- ;ponents A and B, than between the components A and C or B and C.

The new alloys are most advantageously obtained by the methods described from those finely divided metal powders which have been obtained ;from the corresponding metal carbonyls by thermal decomposition.

The following exampleswill further illustrate the nature of this invention but the invention is not-restricted to these examples. The parts are weight.

Example 1 A mixture'of 41 parts of nickel powder, the particles of which have an average size of 8 x strongly kneaded and this promotes interdiifucarbon, and 61.1 parts of iron powder, which contains 1.5 per cent of carbon and 2.2 per cent of oxygen and the particles of which have an average size of 3 x 10- millimetre, is heated, in a vertical mould having the dimensions 150 x 150 x 300 millimetres, for 4 hours at 1100 C., the resulting piece of metal being rolled at the same temperature into a sheet '1 millimetre in strength which is glowed for 3 hours at 750 C. The sheet obtained, which contains 41 per cent of nickel and 59 per cent of iron, when polished and etched has an unhomogeneous structure of the kind already described and has an initial permeability of 900, a permeability of 1000 at a field strength of 0.2 Gauss and a permeability of 1100 at a field strength of 0.4 Gauss, while the same alloy in a homogeneous state has an initial permeability of 2600, and a permeability of 23,000 even at a field strength of only 0.17 Gauss.

Example 2 41 parts of nickel powder obtained from nickel carbonyl and containing 0.5 per cent of carbon and 2.0 per cent of iron, the particles of which have an average size of 4 x 10 millimetre, and 7 parts of cobalt powder obtained from cobalt carbonyl and containing 0.5 per cent of carbon, the particles of which have an average size of 1 x 10- millimetre, are intimately mixed with 55 parts of iron powder obtained from iron carbonyl, the particles of which have an average size of 1 x 10- millimetre and which contains 1.43 per cent of carbon and 2.77 per cent of oxygen, and the mixture is heated, in a vertical mould having the dimensions 150 x 150 x 300 millimetres, for 4 hours to 1200 0., whereby a sintered slug is formed. The material is then forged at the same temperature into a. compact slug of rectangular cross-section which is rolled out at about 1100 C., into a sheet 1 millimetre in thickness. After cooling, the sheet is further rolled to 0.5 millimetre strength and then glowed for 8 hours at 700 C. The resulting material has the following magnetic properties 1. The 'proces of producing magnetic mate rial which comprises consolidating and interdiffusing a mixture comprising 41 per cent of finely divided nickel and '59 per cent of finely divided iron by heating for 4 hours to 1100 C., rolling the mass and glowing it for 3 hours at 750 C.

2. As a new article of manufacture, an alloy:

comprising 41 per cent of nickel and 59 per cent of iron, saidialloy being obtained by the process defined in claim 1.

3. The process of producing magnetic mate rial which comprises consolidating and interdifiuslng a mixture consisting of finely divided perature between 600 and 1300" 0., the duration of the heat treatment being so regulated withregard to the nature of the initial material and.

iron and nickel by heating said mixture at a temthe temperature employed that the mixed metals are only incompletely inter-difiused with each other whereby the micro structure of the resulting alloy is unhomogeneous. p

4. The process of producing magnetic material which comprises consolidating a mixture of finely divided iron and nickel at a temperature above 600 C., to form a body having a physical state varying from porous, when no mechanical pressure is applied, to compact, when mechanical pressure is applied, and then. subjecting the consolidated article to a heat treatment at a temperature between 600 and 1300 C., the duration of treatment being so regulated with regard to employed being higher and the duration of treatment being longer the more porous the consolidated body.

.5. An alloy of iron and nickel having magnetic properties produced by the process defined in claim 3.

6. An alloy of iron and nickel having magnetic properties produced by the process defined in claim 4.

FRANZ DUFISCHMID. 

